Earth's seafloor may be destined to become diamonds

By mimicking how the planet cooks up the sparkly gems, scientists discovered what may be a key ingredient for their formation.

A nine-carat diamond glistens in the light. While such large, clear diamonds are highly prized, our planet hosts a surprising abundance of these precious stones.

Photograph by Todd Gisptein, Nat Geo Image Collection

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These diamonds are so small that more than a hundred can rest easily on the side of a strawberry.

Photograph by Cary Wolinsky, Nat Geo Image Collection

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This cluster of rough diamonds is from Botswana, and its members range in size from a quarter carat to 10 carats.

Photograph by Carly Wolinksy, Nat Geo Image Collection

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A diamond crystal sits on the head of a rivet in this scanning electron microscope image. Its blue coloration is an artifact of how the image was taken and not the natural color of this particular gem. But some diamonds, like the deep blue Hope Diamond, can naturally take on different hues due to minute chemical impurities.

Photograph by James P. Blair, Nat Geo Image Collection

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The colors of these synthetic diamonds were created by irradiation. Colored diamonds can occur in nature, as well, like the yellow of so-called canary diamonds found in places such as Brazil, Australia, and parts of Africa.

Photograph by Cary Wolinsky, Nat Geo Image Collection

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Sorters peer at piles of rough diamonds to divide the tiny gems into classes.

In this lineup of diamonds, only one is real. The fifth ring from the right was cut from a natural diamond; the rest are synthetic.

Photograph by Victor R. Boswell, Jr, Nat Geo Image Collection

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This gold belt ornament is studded with diamonds. While diamonds are prized for their beauty, they also hold many secrets to the processes that churn deep within our planet.

Photograph by Sisse Brimberg, Nat Geo Image Collection

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Expert jewel cutters pose with the largest flawless oval diamond in history, which clocks in at a whopping 102.23 carats.

Photograph by Cary Wolinksi, Nat Geo Image Collection

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This photo of an engraved diamond appeared in a 2002 National Geographic article about the complicated reality of the diamond trade. Engravings are often used to identify a diamond's origin and ensure that it is "conflict free" or "blood free."

Photograph by Cary Wolinksy, Nat Geo Image Collection

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Michael Förster was disappointed. As a Ph.D. student at
Australia's Macquarie University, Förster had spent months cooking rocks
in an attempt to grow the flaky, shimmery mineral mica
in the lab, and he had little to show for his efforts. But in a meeting
with his supervisor, his dismay quickly turned to delight. Instead of unlocking
the mysteries of micas, he realized, the work could explain the enigmatic formation of another sparkly mineral: diamond.

These precious stones often have curiously salty impurities that scientists
have long struggled to explain. The new research, published this week in
the journal Science Advances, suggests that these inclusions are
tiny time capsules of the sediments that once lingered in ancient oceans.

These sediments can get dragged down deep inside Earth thanks to the constant
recycling of our planet's surface at what are known as subduction zones,
regions of the planet where one tectonic plate dives beneath another. The
new work re-creates the complex reactions that take place between 62 and
124 miles beneath the surface, as these sediments and the rocks of deep
Earth stew at soaring temperatures. And the key to these reactions seem
to be ocean sediments.

“I was totally excited,” says Förster, who is now a postdoc at Macquarie.
“I realized how special it was.”

Thomas Stachel,
a diamond scientist at the University of Alberta, notes that this mechanism
may not hold for the more ancient diamonds that formed billions of years
ago on early Earth, when our baby planet was much hotter. But for the younger
diamonds, he says, “this definitely is a very good and interesting explanation.

“I’m not sure this is the final word,” he adds. “But they produced something
that is a very good match by melting the sediments.”

Sparkly innards

Diamonds typically form around a hundred miles below Earth's surface,
crystallizing in what are known as cratonic roots—regions of old, stiff
mantle that prop up overlying continents. So far, the deepest anyone has
drilled into the crust is just over 7.6 miles, so no one has been able
to directly study what happens at these extreme depths.

One particularly curious question is how the chemistry of tiny fluid inclusions
within many diamonds, known as fibrous diamonds, came to be. These fluids
hold an unusual excess of potassium salts relative to sodium salts, chemical
proportions not known to linger deep in Earth.

A 2015 study published in Nature suggested that this salty fluid was the remnant of ancient ocean water that
had altered the seafloor, becoming entrapped in the minerals of the crust
and later released when the slab of subducting rock plunged into Earth.
But this still couldn't explain the unusual excess of potassium in the
diamond's salty slurries, Förster says.

Deep Earth in a pill

In the latest work, the researchers re-created the conditions that might
be encountered deep underground in a tiny platinum capsule. This tin can-shaped
container was lined with carbon and then filled with a layer of ground-up
ocean floor sediments recovered from the International Ocean Discovery Project,
along with a layer of ground-up minerals of a rock known as peridotite,
which is common in the upper mantle where diamonds form.

Then the researchers used a piston cylinder to compress the small capsule
and reach the pressures of diamond-forming zones, up to six gigapascals—akin
to “a whole building standing on your foot,” Förster notes. Finally, the
capsule was electrically heated to attain temperatures as high as 2,012
degrees Fahrenheit and then allowed to brew for between two and 14 days.

I Didn't Know That: Diamonds

When the capsule's baking was complete, the researchers studied the results
of the chemical reactions. They discovered a high ratio of potassium to
sodium salts, similar to that found in the diamond inclusions, as well
as the formation of a sodium-rich mineral known as clinopyroxene.

“It’s essentially this formation of clinopyroxene that soaks up the sodium
in this reaction zone,” Stachel explains. That same reaction may be happening
deep underground, allowing the diamond inclusions to achieve their curious
chemistries.

The birth of a gem

Armed with this new information, the researchers now propose a tweaked
model for how many of the world's known diamonds were born.

When a tectonic plate is shoved into the deep, they say, an earthy soup
begins to form. Water is released as the minerals of the crust and clays
from sediments are heated. The water becomes infused with dissolved carbon
from organic material on the ocean floor, the mantle itself, or carbonate
sands made from limestone and the skeletal remnants of ocean critters.

The newly proposed component, the melty ocean sediments, also get added
to the mixture. This fluid then percolates up through the mantle, reacting
with the surrounding rocks to produce the final carbon-rich, salty solution
from which diamonds slowly crystallize.

Karen Smit,
a diamond geologist at the Gemological Institute of America, is excited
about the new work but cautions that there is still plenty to learn about
how these gems form.

“This is happening in an inaccessible part of the Earth, and there’s just
still a lot of unknowns out there,” she says.

Smit also emphasizes that not all diamonds form in the same way. Some
diamonds seem to grow super-deep, at hundreds of miles below the surface.
Some even contain bonus elements like boron that impart brilliant colors,
as seen in the Hope Diamond's famous azure hue.

“Every day we see things that we don't understand,” Smit says, noting
that her work at the GIA brings her into contact with hundreds of diamonds
each day. Still, the new research is an exciting clue to add to our understanding
of diamond formation—and it underscores an oft overlooked significance
of these glimmering stones.

“Diamonds aren’t just pretty,” she says. “They also tell us a lot about
the Earth.”

Editor's note: This article has been corrected to clarify the depth of
the deepest hole ever drilled.

PUBLISHED May 30, 2019

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